1. Field of the Invention
The present invention relates to an image sensing apparatus having a distance measuring unit, and a control method thereof.
2. Description of the Related Art
A digital camera has an image sensing element such as a charge coupled device (CCD) for photographing. As an auto focus (AF) scheme using such image sensing element, an AF scheme that makes focus adjustment of a photographing lens by effectively using an image signal output from the image sensing element upon infinitesimally displacing the photographing lens of the digital camera is known. Such an AF scheme will be referred to as an imager AF scheme hereinafter. The imager AF scheme determines the focal point position of the photographing lens by detecting the contrast of an image signal output from the image sensing element.
On the other hand, a silver halide camera (especially, a compact camera) does not use such an image sensing element upon photographing. For this reason, the compact camera incorporates an AF distance measuring unit in an optical system independent from the photographing optical system including a photographing lens. That is, a compact camera normally adopts an AF scheme which controls the photographing lens in accordance with an output signal from the distance measuring unit. Such an AF scheme will be referred to as an external light AF scheme hereinafter.
In recent years, it is understood that these imager and external light AF schemes suffer drawbacks. The drawbacks of these AF schemes are as follows.
The imager AF scheme basically attains AF using an image sensing element for photographing. For this reason, when the photographing lens is largely defocused from an object, AF control using the imager AF scheme cannot be made. When the photographing lens is largely defocused from an object, the image sensing element cannot acquire a clear image of the object. The imager AF scheme can determine the focusing direction only after the photographing lens has moved. Furthermore, the imager AF scheme is limited in capture time of an object image since it attains AF while moving the photographing lens. For this reason, the imager AF scheme requires a long focus adjustment time. In addition, when the imager AF scheme is used, it is difficult to detect a principal object by comparing the distances to objects present at a plurality of points.
On the other hand, the external light AF scheme allows high-speed distance measuring since it uses a sensor dedicated to AF. However, the external light AF scheme cannot cancel errors such as position errors of the photographing lens due to changes in environment such as temperature and humidity; errors depending on the photographing posture or the like; and similar errors, since it does not execute feedback control of the photographing lens.
In order to eliminate these drawbacks, Jpn. Pat. Appln. KOKAI Publication No. 2000-321482 discloses a technique that uses the imager and external light AF schemes in combination. That is, the technique disclosed in this reference executes coarse distance measuring using the external light AF scheme, and then final focus adjustment using the imager AF scheme.